[1]} analysed the photometric properties of central galaxies in FGs. They studied the position of the central galaxies in FGs in the fundamental plane and its projections to explain the formation of these objects (see Fig. REF ). Central galaxies in FGs results to have large \(K\) -band luminosities. The \(K_s\)  luminosity is a good proxy of the total stellar mass since the typical M/L \(\sim 1\)  for an old stellar population [2]}. Thus, it seems clear that central galaxies in FGs are amongst the most massive galaxies known. The left panel of Fig. REF  shows the correlations between the \(K_s-\) band luminosities and the central velocity dispersion [3]}. Similarly, the right panel of Fig. REF  shows another projection of the fundamental plane, the \(K_s-\) band luminosity vs. effective radius (\(r_e\) ). In both relations BCGs in FGs are found slightly outside the correlations: this bend was found for the first time in [4]} for early-type galaxies and it can be interpreted in terms of the formation scenario of the BCGs. In particular, if major dissipationless mergers between galaxies are the main mechanisms to build up the mass of the BCGs, the final size is expected to increase, but not its central velocity dispersion; however, if minor dry mergers are the predominant mechanism, they are expected to change both the size and velocity dispersion of the BCGs. Thus, the results presented in [1]} seems to favour the first scenario for these massive galaxies.
This is also supported by new results obtained from numerical simulations. [6]} investigated the origin of FGs in the Illustris simulation. With respect to the stellar mass assembly of the BCGs, they found (see their fig. 6) that the accretion is similar at high-redsfhits for fossil and non-fossil systems, whereas a clear difference start to appear at \(z \sim 0.3\) . From this point, BCGs systematically accreted more mass in FGs than in non-FGs.
